The present invention relates to a brake adjustment mechanism for adjusting a vehicle brake for wear, and more particularly to an automatic brakeshoe adjustor assembly to accommodate wear of brake linings on cam actuated air brakes of the type commonly employed on large heavy duty vehicles, such as trucks and truck trailers.
The air brakes that are used on heavy duty vehicles generally include an air brake chamber to which air is supplied when the vehicle is subjected to braking. When pressurized air is admitted to the brake chamber, a braking force is transmitted to a cam shaft via the action of a push rod and brake lever. Once the cam shaft is rotated, the cam at the end thereof, commonly an S-cam, simultaneously rotates against a roller engaged at both ends of the brakeshoe webs. This actuation on the respective rollers causes the brakeshoes to be spread apart forcing the brakeshoe lining against the brake drum, thereby causing the wheels of the vehicle to stop.
The brakeshoes generally employed on these types of vehicles can be of two types: a double web and/or a single web brakeshoe. The double web brakeshoe is used for brakes having a greater surface area which generally require a wider shoe and brake lining, and therefore necessitate a greater support in the form of a double web. This form of the brakeshoe is utilized on wheels of trailers and truck vehicles whose loads are substantial. The roller assemblies engaged at the ends of these webs for actuation by the cam shaft are generally made up of a hollow roller embodying a concentric pin axially extending from the roller along the roller's rotational axis, such as those set forth on pages 1-2 to 1-14, 1-18 to 1-30, and 1-52 to 1-54 of the Euclid Brake Parts Manual. The rollers are disposed between the ends of the double webs by having the pins engaged in semi-circular or circular openings positioned at the ends of the webs as illustrated in the parts drawings of the Euclid Brake Parts Manual.
The single web brakeshoe supports a lesser braking surface area for the brake lining and is generally used on the front of the truck where substantial weight loads are not a factor. Accordingly, these brakeshoes use a single web for supporting the shoe and corresponding brake lining. The type of roller assembly employed for actuation by the cam shaft usually consists of a pair of rollers on either side of an end portion of the single web and connected to each other by a pin concentrically fixed to each of the rollers. The exposed portion of the pin between the rollers is engaged in a semi-circular or circular opening in the end portion of the web, as illustrated on pages 1-2 and 1-16 of the Euclid Brake Parts Manual.
The foregoing braking systems used on heavy duty vehicles typically include some form of mechanism that will compensate for and progressively take up the slack that is generated by wear of the brake linings. An adjustment is needed because the clearance that normally exists between the brake drum and brakeshoe lining will eventually increase beyond a preset limit due to the wear of the brakeshoe lining. When these brakeshoe linings wear, the push rod of the air brake chamber has to be extended or pushed further to rotate the cam shaft against the brakeshoes to compensate for the increased clearance. The Department of Transportation imposes strict requirements for the clearance limits depending on the size and configuration of the braking system employed.
Common forms of brake adjustors generally employed include slack adjustors, which are usually positioned between the push rod and the cam shaft. Other forms include self adjusting brake adjustors that operate more directly at or near the brakeshoe(s) to compensate for wear. The present invention falls within this latter class of brake adjustors, and typical disclosures may be found in U.S. Pat. Nos. 1,875,064 and 1,875,065, both issued to Lyman (1932); U.S. Pat. No. 2,522,903 issued to Shively (1950); and U.S. Pat. No. 4,586,589 issued to Idesawa (1986). Unfortunately, the mechanisms disclosed therein contain a multitudinous number of parts, require extensive assembly and disassembly when the brakes are subjected to routine maintenance, and require extensive and frequent maintenance themselves by the replacement of worn, misaligned or failed parts which can lead to a costly and time consuming practice. These factors render the foregoing devices impractical for extensive and reliable use.
What is needed then is a self adjusting brakeshoe adjustor that consists of a minimum number of parts, is relatively inexpensive to manufacture, and which is dependable and reliable in its construction. It is also desirable that such a device be easily incorporated into existing braking configurations, installed or removed without disassembly, one that requires minimal maintenance, and optimally, one which can operate with or without the existence of an automatic slack adjustor.
The foregoing desirable characteristics of a self adjusting brake adjustor are accomplished by the invention herein which is described below.